1. Field of the Invention
The present invention relates to a record medium checking device for detecting the existence of defects such as dropout, pinhole or the like generated on recording mediums such as magnetic tape, magnetic discs or the like.
2. Background Information
In the prior art, a dropout, a pinhole and the like on recording mediums are detected by a method comprising the steps of recording a signal on the recording medium, and comparing the level of the reproduced signal recorded on the recording medium with that of a predetermined reference signal.
A device for carrying out the above described detection is comprised of a signal generating means for generating a signal to be recorded on a recording medium; recording means for recording the signal on a recording medium; reproducing means for reproducing the recorded signal; and an AGC circuit for controlling the level of the reproduced signal to an average level and for outputting the level control signal. The device also includes full-wave rectifying means for full-wave rectifying the level control signal; a peak detecting circuit having the drooping time constant for detecting a peak of the full-wave rectified signal; and discriminating signal output means for outputting a discriminating signal when the output from the peak detection circuit is lower than a threshold level.
In the above described device, when there is a dropout on a recording medium, attenuation of the output signal form the peak detecting circuit between a first peak and second peak, which depends on the drooping time constant, continues so that the level of the output signal is lower than a threshold level. Thus the device detects the dropout.
Further, Japanese Patent Publication No. 49310/1988 (Showa 63) discloses a dropout detection device in which pulses synchronized with zero crossing points of the signal reproduced from a recording medium are generated and includes a detector circuit comprised of an one shot multivibrator. If the pulses are constantly generated, the detector circuit keeps its output at a high level. On the other hand if the pulse train is interrupted because of a dropout, the output of the detector circuit changes to a low level, thus the dropout is detected.
When signals with low frequencies are recorded on recording mediums for inspection, the above devices can operate without a problem. However, when signals with very high frequencies, for example, signals of about several MHz are recorded on recording mediums for inspection, stable operation of the devices is difficult, because a full-wave rectifying circuit, a peak detecting circuit having the drooping time constant, and a multivibrator must be used.
Further, when the frequency of the signal to be recorded is changed according to the transport speed of a recording medium, alteration of specifications etc., the time constant of the peak detecting circuit or one shot multivibrator must be modified, therefore the circuits of the device for the modification of time constants are bulky.